1. Field of the Invention
The present invention relates to a rectifier using an autonomous synchronous rectification MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and an alternator and a power source using the rectifier.
2. Description of the Related Art
For an alternator for generating electricity in a vehicle, diodes have been used as a rectifying element. The diode has a large loss due to a large forward voltage drop in spite of a low cost. On the other hand, a MOSFET begins to be used as a rectifying element for the alternator in place of the diodes. Use of the MOSFET for synchronous rectification provides a rectifying element having a small loss without a large forward voltage drop in which the forward current increases from zero volts.
As a method of controlling the synchronous rectification MOSFET for the alternator between on and off, the method of controlling the MOSFET by detection of a position of a motor with a Hall element has been known. Such a method of performing control using an input signal from the external with the Hall element is called here an External control type. The external control type of the synchronous MOSFET needs to use a sensor such as the Hall element and it is necessary to perform a complicated control with a control circuit, so that a rectifying part of the alternator becomes expensive.
In Patent Document 1 (JP2009-524403), a column of problem in abstract describes “to provide a control device for a transistor to block a leak current”, and a column of Solving means describes as follows:
To control a gate of the transistor by an output control signal, an amplifying device (15) is provided which includes a first input connected to a drain of the transistor for forming a first connection overall and a second input connected to a source of the transistor to form a second connection overall. Further at least one protection device is provided which includes at least one switching device (T1) serially inserted in the first connection of the amplifying device (15) and a generation device configured to generate a regulation voltage to control the transistor and to allow the same number of semiconductor junctions to exist in the first and second connections. This is applicable to a charging device for a battery.    Patent Document 1: JP2009-524403
There are two types of comparators for determination between on and off of a MOSFET in accordance with a drain-source voltage, namely, a bipolar type of comparator configured with bipolar transistors and a C-MOS (Complementary MOS) type of a comparator configured with MOSFETs. When the comparator of the C-MOS type is used, though an input offset voltage cannot be decreased too much, a consumed current can be decreased. Smaller consumed current of the comparator decreases a consumed current in a control circuit and decreases capacitance and size of a capacitor supplying a power source voltage to the control circuit.
When the comparator of the C-MOS type is used, an input terminal of the comparator is connected to a gate of a MOSFET included in the comparator. Two input terminals of the comparator are connected to gates of the MOSFETs included in the comparator inside the comparator and to a source and a drain of the synchronous rectification MOSFET outside the comparator. A drain of the synchronous rectification MOSFET is supplied with a high voltage of a battery connected to an alternator, and the high voltage is applied to a gate of the MOSFET connected to the input terminal of the comparator. Application of the high voltage to the gate of the MOSFET causes deterioration of the gate insulation film such as HCI (Hot Carrier Injection), TDDB (Time Dependent Dielectric Breakdown), and PBTI (Positive Bias Temperature Instability), which may change a threshold voltage of the MOSFET.